Practical investigations
Read our general notes on Risk Assessment
Investigating vinegars
This tutorial responds to the many questions from students asking about investigations involving the analysis of vinegars.
If you are following the Nuffield course you are expected to plan you own investigation, so here we make suggestions and offer hints. We do not give all the answers.
Some of the other tutorials in this section of Re:act may also be helpful with planning and when it comes to assessing the results.
We suggest a wider range of techniques than you need to carry out for achieve success in a Nuffield A2 investigation. Check with the marking criteria in the course specification from Edexcel to ensure that your investigation and report cover all the aspects which your teachers and the examiners are looking for.
Note that in this topic the manufacturers and users of vinegars generally use common rather than systematic names for chemicals and we tend to do the same.
See also our sorted FAQs on vinegar investigations
WHAT IS VINEGAR?
Vinegar comes from the bacterial oxidation of alcoholic drinks such as wine and cider, and the formation of ethanoic (acetic) acid gives vinegar its characteristic smell. However, ethanoic acid is by no means the only acid in vinegar. It is not even the most predominant acid - it is simply the one, which, owing to its volatility, makes its presence known! Many other acids are also present, such as tartaric, malic, lactic, citric and succinic acids. There are probably many more besides, but since these latter acids are non-volatile they have no smell, yet they still contribute to the overall acidity of the vinegar. You will need to look up the structures of these non-volatile acids.
Vinegars also contain other components, such as esters, and possibly small amounts of sugars, so an extended investigation might cover the analysis of these components too (see the tutorial on “Wines”).
There are also vinegars made from synthetic chemicals. These are less complex. Non-brewed condiment, for example, is a vinegar substitute made from food-grade, industrial ethanoic acid.
SUITABLE VINEGARS
Possibly the best known type of vinegar used in this country is malt vinegar, made by the two-fold fermentation of barley malt or of other cereals where starch has been converted to maltose. However, there are many other types of vinegar: red and white wine vinegars, cider vinegar, and several more as well, and you will almost certainly want to use at least two different types in your investigation, possibly more. Spirit or distilled vinegar made from the bacterial oxidation of dilute distilled ethanol is a ‘simpler’ mixture, and its acidity is therefore likely to be due solely to the presence of ethanoic acid, so perhaps this might be a good choice of an additional vinegar for comparative purposes.
DECOLOURIZATION OF COLOURED VINEGARS
You are likely to be using techniques involving colour changes, so coloured vinegars are going to mask these and pose problems. You are therefore going to need to decolourise your vinegar first. (HINT: activated charcoal should come in handy)
ANALYSIS OF ACIDITY CONTENT
The obvious method to use is a titration, but you might also consider chromatography too. A pH probe could be used to estimate the relative acidities of different vinegars, but can only yield limited information as regards the absolute concentrations of the acids in the vinegar.
[1] Titration methods
(a) Practical work
Clearly you will need to choose an appropriate alkali for neutralization which will have to be a strong base to titrate with weak acids. Unfortunately the common strong bases react with the carbon dioxide in the air, so their purity is questionable. They are not suitable as primary standards. You cannot be sure of the concentration when you make up solutions of strong alkalis using a graduated flask.
Therefore, for accurate work, and if you want to impress – you will have to standardise your solution of alkali solution before using it to titrate your vinegar samples. To do this you will have to find a reliable acid as standard. There are several of these (for example, potassium hydrogenphthalate).
Before starting you will need to carry out trials, using perhaps a teat pipette, test tube and indicator to estimate the approximate concentration in moles per litre of the acids present in the vinegar. You should now use a solution of alkali of approximately the same concentration (why?) having standardized it first with a solution of your ‘reliable’ acid.
As mentioned earlier, there are many different acids present in the vinegar, so this titration will obviously account for all of them. You might now consider distilling a known volume of the vinegar. How do you think you will minimise the loss of the volatile acids distilling over in this process, and how do you think you will know when all or most of the volatile acids have been driven off? You can subsequently carry out a titration on the distillate, and this will give a measure of the acidic content of the vinegar due to the volatile components (mostly ethanoic acid). Finally you could carry out a titration on the remaining boiled vinegar to give an estimate of the total non-volatile acid components.
We have been asked about the likelihood of the acids denaturing during the boiling process. We think that while this could cause some of the other components to decompose or oxidise, the acids are less likely to be affected.
(b) Calculations
The main problem is that you are dealing with mixtures of acids rather than a single one, but the convention here is to quote values in terms of one of the acids (usually the one which predominates).
In the case of the volatile acids, ethanoic acid probably accounts for most of the acidity present, and tartaric acid is often regarded as the non-volatile acid in greatest abundance. Incidentally, you will need to look up the formula for tartaric acid (butanedioic acid) and work out the equation for its neutralization with the alkali you have chosen.
It is conventional in food science to quote your concentrations in g dm-3 rather than in mol dm-3.
[2] Chromatography
(a) Practical Work
Paper chromatography is the best option in a school or college laboratory, and you might like to consider using the procedure described in Topic 18 of the Nuffield Students’ Book.
You need to obtain pure samples of the acids most likely to be present in the vinegar (tartaric, malic, lactic and so on) and use solutions of these as reference spots alongside those from your vinegars. Since all the acids are soluble in water (why, from an intermolecular point of view, is this true?), this solvent is probably a good one to use. Once the chromatogram has been removed and is dry you will need to spray the paper with an acid-base indicator, so that the acids will show up in an appropriate colour against a contrasting background of the other colour that the indicator shows at higher pH. Try bromophenol blue or bromothymol blue for example, and see which one appears to work best. For other alternatives refer to Table 6.6 of the Nuffield Data Book. An aqueous solution of the indicator contained in a polythene house plant spray bottle works well.
You will probably have to repeat this procedure several times to achieve a good technique and useful results, but the effort is well worth it.
(b) Calculations
Finally you can measure RF values and/or compare the spots produced by your acids against the reference acids alongside.
This technique will give you a rough qualitative analysis of some of the acids present, but cannot easily be adapted to give quantitative results. For this you need to use gas chromatography which is probably unavailable in your chemistry laboratories. However, the more enterprising of you may be able to make contact with a local analytical lab and arrange for a suitable analysis to be carried out, preferably in your presence. You should get credit for this as part of your background research.
REFERENCE MATERIAL
The acidity of a vinegar is occasionally stamped on the side of the vinegar bottle. Also you could contact one of the supermarkets (preferably the one from which the vinegars were bought) to obtain ‘official’ results of the analyses which they carry out from time to time on their range of vinegars. This would be a useful means of comparing your results with theirs, and of attempting to account for any differences you find.
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che'nor
This article is very useful while doing my Project
27 July 2006
Vick
Really useful info to add that final pizazz to my practical report. Not exactly a method but gives an idea of the assumptions which need to be made and improvements which can be made to an existing prac.
27 March 2006
PETE
WOW! this is amazing I didn't have a clue what to do and where to get started this has given me enough info to know how to get started and finish!!!
EXCELLENT!!
CHEERS RE:ACT
23 March 2005
SHAZ
This was really good it gave me an idea of how to carry out my practical
09 October 2004
ALEX
This was a real help on helping me to decide what to do in my practical investigation.
06 October 2004
Louise
Fab! Thanks. I hadnt a clue where to start on my investigation and this has given me some really good ideas.
13 November 2003
updated: 12 March 2008
